The present invention relates to a technological process for the preparation of lower olefins with stress on ethylene by conversion of hydrocarbons.
Research on the production of ethylene by the pyrolysis of heavy oils has been very active in recent years both in china and abroad, for example, the QC (quick contact) reaction system developed by Stone and Webster Eng Co. of USA (U.S. Pat. No. 4,663,019, ZL88102644.1 and EP 0381870A etc.): this technology uses a downward tubular reactor and a feeding mode of a single feed oil. The steam pyrolysis technology developed by British Petroleum Ltd. (U.S. Pat. No. 4,087,350): this technology uses a tubular fixed bed catalytic reactor. The technology developed by Tokyo Science and Technology Co. of Japan for producing olefins using coke particles as a heat carrier (U.S. Pat. No. 4,259,177): this technology uses a fluidized bed reactor of the reaction-regeneration type. The HCC (Heavy-oil contact cracking) technology developed by SINOPEC Loyang Petro-Chemical Engineering Co. (ZL 92105507.2): this technology uses an up flow or down flow tubular piston flow reactor and a feeding mode of a single feed oil. The above technologies are all able to produce ethylene from heavy oils. But in the flow sheets of the above technologies, only the case of feeding a single fresh feed oil is considered. American patent U.S. Pat. No. 5,348,644 is patent for an invention relating to the improvement of the feeding equipment and technological process of a lift pipe catalytic cracking device. The fluidization state of the catalyst in the lift pipe is optimized by installation of special equipment in the pre-lift section of the lift pipe and thereby the contact state of the catalyst and the feed oil for the catalytic cracking in the lift pipe is improved and more ideal product distribution of the catalytic cracking is obtained. Chinese patent ZL 8910052, U.S. Pat. No. 5,264,115 and U.S. Pat. No. 5,506,365 provide a fluidized bed process and device for converting hydrocarbons which consists of a steam pyrolysis section for light hydrocarbon fractions at the upstream of a reaction zone and a catalytic cracking section for heavy hydrocarbon fractions at the downstream of said reaction zone in a tubular reaction zone with an upstream or downstream flow in the presence of the catalyst particles in a fluidized phase. The applied catalyst belongs to the type of catalytic cracking. The purpose is to obtain a propylene yield slightly higher than the conventional catalytic cracking while raising the yields of gasoline and diesel oil in the product. The major characteristic is to separate C2 components from the product and then introduce them into an oligomerization reactor to proceed the oligomerization reaction; the remained C2 components and the oligomerization products are returned to the lift pipe to proceed the steam cracking reaction so that the purpose of raising the yields of fuel oils and propylene is achieved. It can be seen from the example that the yields of the C2 olefin, C3 olefin in this technology are both lower than 7.0% by weight.
In order to solve the problems of simultaneous feeding of multiple feed oils and/or re-refining of some pyrolyzed by-products (e.g etheane, propane etc) in the technology of heavy oil pyrolysis to prepare ethylene, the present invention is to provide an effective method which allows different feed conduct the proylsis under different process conditions so that optimizing the reaction conditions and product structure, raising the yield of ethylene and saving the capital and operation costs can be realized.
The major characteristic of the present invention is the multiple feed accompanied by ethane re-refining for the purpose of producing more ethylene. In the technological process for conversion of hydrocarbons to prepare gaseous olifins by bringing them into contact with solid catalyst particles, the feed hydrocarbons are not only one and the desired pyrolysis conditions for various hydrocarbons are not completely the same. For example, the optimal reaction temperature for ethane is higher than that for naphtha and the optimal reaction temperature for naphtha is highter than that for vacuum distillates, and so on. In order to pyrolyze various feed hydrocarbons under their respective optimal conditions as far as possible, separate heaters are used in the tubular heater pyrolysis technology. For example, there must be one or two ethane pyrolysis heaters to proceed pyrolysis for re-refining ethane in the plant that uses naphtha or light diesel oil as a feed. In the technology of the catalytic pyrolysis for ethylene preparation, it is impossible to pyrolyze the feeds with different properties in separate heaters because there is only one reactor. The optimal pyrolysis conditions can not be met for various hydrocarbons if they are mixed and then fed. Taking the simultaneous pyrolysis of an atmospheric residue and ethane as an example, the optimal pyrolysis temperature for the atmospheric residue (at the outlet) is 650-750xc2x0 C., but the pyrolysis rate of ethane in this temperature range can not meet the need of industrial production; if the pyrolysis temperature is raised to above 800xc2x0 C. to meet the conditions for ethane pyrolysis, the pyrolysis extent of the atmospheric residue can not be controlled. In the above case, it is possible to feed the atmospheric residue and ethane separately and attain desired pyrolysis extents for various feeds by using the method of the multiple feed at separate points of the present invention.
One aspect of the present invention is therefore to provide a process for catalytic pyrolysis of hydrocarbon feeds to produce lower olefins with stress on ethylene and co-produce light aromatics, which is to bring the hydrocarbon feeds into contact with a solid granular catalyst in a piston flow reactor to proceed catalytic pyrolysis, the hydrocarbon feeds include two or more hydrocarbons having different physicochemical properties, the feed hydrocarbons are mixed with steam and introduced, the general reaction conditions in the reaction zone are: temperature 600-900xc2x0 C., pressure 0.13-0.40 MPa (absolute), total steam/hydrocarbon ratio 0.1-1.0, total catalyst/oil ratio 5-100 and the catalyst/oil contact time 0.02-5 s; the oil gas after reaction is separated quickly from the catalyst and quenched, the catalyst is recycled for reuse after regeneration, different feeds are introduced from different positions, hydrocarbons difficult to pyrolyze are first introduced into the reactor and brought into contact with the catalyst of high temperatures and high activities from the regenerator and the pyrolysis takes place, meanwhile, the catalyst cools down and deactivates; then other hydrocarbons easier to pyrolyze are introduced in sequence from the upstream to downstream of the reaction zone, the hydrocarbons introduced later play a role of quenching those introduced earlier, the temperature of the reaction zone and the activity of the catalyst are lowered step by step from the upstream to the downstream; the positions from which various hydrocarbons enter the reactor are determined as such that the residence times of various hydrocarbons in the reactor are gradually decreased in the sequence from difficulty to ease in pyrolysis, the differences in the residence times of every adjacent two hydrocarbon feeds in the reactor are 0.01-3 s.
Another aspect of the present invention is to provide a process for direct conversion of heavy hydrocarbons to produce lower olefins with stress on ethylene and co-produce light aromatics, which is to bring the hydrocarbon feeds into contact with a solid granular catalyst in a piston flow reactor to proceed catalytic pyrolysis, the feed hydrocarbons are mixed with steam and introduced, the general reaction conditions in the reaction zone are: temperature 600-900xc2x0 C., pressure 0.13-0.40 MPa (absolute), total steam/hydrocarbon ratio 0.1-1.0, total catalyst/oil ratio 5-100 and the catalyst/oil contact time 0.02-5 s; the oil gas after reaction is separated quickly from the catalyst and quenched, the catalyst is recycled for reuse after regeneration; the oil gas enters a fractionation and separation system to proceed the separation, a product gas mainly containing ethylene, the by-product ethane, and a liquid product rich in aromatics can be obtained, the highly pure by-product ethane from the separation system and/or the gases containing ethane from other sources return to the pyrolysis reactor of the piston flow type from the upstream inlet of the reactor and come into contact with the catalyst of high temperatures and high activities, fast pyrolysis takes place at temperatures higher than 780xc2x0 C. to produce ethylene and meanwhile, the catalyst cools down and deactivates, steam is introduced at the same time when ethane feed is introduced from the upstream inlet of the reactor; heavy hydrocarbon feeds are introduced at position certain distance from downstream of the inlet for ethane, the hydrocarbons introduced later play a role of quenching those introduced earlier, the reaction temperature at this moment is lowered to 680-800xc2x0 C.; the feeding positions of the by-product ethane and the heavy hydrocarbons are determined as such that the residence time of ethane, which is difficult to pyrolyze, is long, while that of heavy hydrocarbons, which are easy to pyrolyze, is short, the differences in the residence times of ethane and heavy hydrocarbons in the reactor are 0.01-3 s.